1. Field of the Invention
The present invention relates to bar code scanning systems, and particularly to optics utilized in the projection of a laser scanning beam towards a bar code for controlling the width of the beam in the scanning direction over a significant range away from the scanner where bar codes may be located for reading.
The system is especially suitable for use in an optical assembly for a bar code scanner comprising a laser, for example, a laser diode, and optical elements which form and project the beam generated by the laser so that the beam, across its waist and over a range of distances where bar codes may be located for scanning and reading, is less than the width of the code, so as to enable bars and spaces of the code to be resolved from the return light from the code which is detected by a photodetector in the bar code scanner thereby optimizing scanner performance for a predetermined minimum bar or space width. This invention may be used to optimize scanner performance over desired reading range in an application for which the minimum bar and space widths are known. For example, a significant range may be a distance of about 30 inches from the scanner for a typical UPC product code with minimum bar and space widths on the order of 10 mils.
It is a feature of the invention to provide in an optical assembly which launches and projects the scanning beam via optical elements which form and control the phase of the beam as it is launched so as to control and limit the size of the beam at least across its waist (as measured along the scanning direction) over a significant scanning range in front of the scanner.
2. Description of the Prior Art
Phase masks and particularly masks having apertures have been used to condense optical beams so as to provide high resolution at locations close to the mask. Such phase masks with circular or rectangular apertures have been proposed for optical disc recording and reading to provide so-called "super resolved" beam spots. See Hirose, et al., U.S. Pat. No. 5,121,378, issued Jun. 9, 1992. Phase plates have been proposed to provide phase or amplitude changes for coalescing lobes of a laser beam pattern and enhancing intensity of laser beams. See an article by L. W. Casperson, which appeared in Laser Focus World magazine for May 1994, pages 223-228 which describes various phase mask or plate systems. In order to control the size of a scanning beam over a large, significant range in front of the scanner, it has been proposed to utilize apertures with predetermined sizes and in predetermined space relationships with the source of the laser beam. The apertures may have different transmissivity in different regions thereof. See, Eastman, et al., U.S. Pat. No. 5,200,597, issued Apr. 6, 1993 and Eastman and Quinn, U.S. Pat. No. 5,386,105, issued Jan. 31, 1995. Special lenses called axicons which focus apodized laser beams have also been proposed in order to control the size of a laser scanning spot. See Marom, U.S. Pat. No. 5,331,143, issued Jul. 19, 1994. Multiple overlapping beams have been also proposed. See Marom et al. U.S. Pat. No. 5,315,095, issued May 24, 1994.